At least one person has died and more than 450 others have been hospitalised due to a mystery illness that has broken out in southeastern India.

Indian health authorities are baffled by the reason behind why large numbers of residents in the southern state of Andhra Pradesh are collapsing and experiencing seizures and nausea.

Widespread water contamination and COVID-19 have been ruled out as the cause.

District surveillance officer Dolla Joshi Roy said at least 455 people have been admitted to hospital in the town of Eluru, with symptoms including nausea, shivering, anxiety, seizures and some passing out.

A 45-year-old man suffering from nausea and epilepsy-like symptoms had died just hours after being hospitalised on Sunday, the Press Trust of India news agency reported.

But state health spokesman Manikyala Rao said the death was due to a heart attack and not related to the illness.

About 200 patients have been discharged since the illness was first detected in Eluru on Saturday evening.

The Indian Express quoted a medical officer from Eluru hospital as saying “the people who fell sick, especially the children, suddenly started vomiting after complaining of burning eyes. Some of them fainted or suffered bouts of seizures”.

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District surveillance officer Mr Roy said specialist doctors are still trying to determine the cause of the sickness.

“It could be food or waterborne or airborne or some other condition. But we do know this was very localised, the people affected all lived in the same city,” he said.

He said they were awaiting the results for E-coli, which originates in the stomach and is acquired by coming into contact with infected faeces.

So far, water samples from affected areas haven’t shown any signs of contamination and the chief minister’s office said people not linked to the municipal water supply have also fallen ill.

The patients are of different ages and have tested negative for COVID-19 and other viral diseases such as dengue, chikungunya or herpes.

The outbreak is proving a significant challenge for the healthcare system in the state of Andhra Pradesh, which has 800,000 active coronavirus cases.

An expert team deputed by the federal government reached the city to investigate the sudden illness on Monday.

State chief minister YS Jagan Mohan Reddy visited a government hospital and met patients who were ill.

Opposition leader N Chandrababu Naidu demanded on Twitter an “impartial, full-fledged inquiry into the incident”.

Andhra Pradesh state is among those worst hit by COVID-19, with more than 800,000 detected cases.

The health system in the state, like the rest of India, has been frayed by the virus.